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June 14, 1960 .E. K. KAPRELIAN 2,940,847

ELECTROPHOTOGRAPHY Filed July 3, 1957 2 Sheets-Sheet 1 G H 6 '4 3 w o w5 m H u. 5 4 "HW 2 4 m F 5 WW. 1 WE G 0 P w E L i if.

ADNE RED BLUE GREEN WHITE June 14, 1960 E. K. KAPRELIANELECTROPHOTOGRAPHY 2 Sheets-Sheet 2 Filed July 3, 1957 NONE RED GREENBLUE WH/TE F/GJ4 INVENTOR. @MK

United States Patent O 2,940,234"! ELECTROPHUIOGRAPHY Edward Knprelian,College Highway,

' Weatogue, Conn.

Filed July 3, 1957, Ser. No. 669,866 7 Claims. (Cl. 96-1) This inventionrelates to improved methods and means for electrostatic colorphotography or color printing.

In ordinary electrostatic photography or printing there is formed on aninsulating surface an intermediate electrostatic image, corresponding inpotentials to the light values of the original object, and thiselectrostatic image is rendered visible by dusting with a suitablepowder which adheres selectively to the surface in a patterncorresponding to the electrostatic image. A description of this processmay be found in US. Patent 2,297,691, issued October 6, 1942, .to C. F.Carlson.

It is also possible to practice electrostatic photography byilluminating a layer of normally insulating photoconducdve powderwhichis located in an electrical field. In this case powder lying in anilluminated area becomes charged and is attracted away to a region ofopposite polarity. description of this process may be found in U.S.Patent 2,758,939,-issued August 14, 1956, to M. .L. Sugarman.

in the electrophotographic patents of the prior art the an ordinarymonochromatic processes of these and other action is essentially that ofor black-and-white system.

The basic imageis one rendered in monochrome, and

it is possible, by utilizing color separation techniques, to producecolor prints or photographs by superimposing in properregistryseparation images employing properly chosen dyes or pigments.

In contrast with previously known systems the-present invention producesthe color image directly in a single step without the use of separationimages. In the practice of this invention the color image is produced bythe selective migration of charged color particles in an electriclalfield according to the color or wavelength of the ig t.

One of the objects of this invention is to employ the principles ofelectrostatic electrophotography in the production of color prints.

Another object is to provide a relatively simple, direct and low costarrangement for the production of color photographs, prints, posters andsigns.

Still another object is the provision of a method and means for thecontinuous production of color prints.

These and other objects will become the specification and drawings inwhich Fig. 1 shows in cross section one form of photoconductive colorparticle.

Fig. 2 shows in cross section another form of photoconductive colorparticle. "Fig. 3 shows in cross section still another form ofphotoconductive color particle.

Fig. 4 shows in cross section still another form of photoconductivecolor particle.

Fig. 5 shows in cross section one conductive color particle.

Fig. 6 shows in cross section another form of nonphotoconductiveparticle.

Fig. 7 shows diagrammatically the arrangement of photoconductive colorparticles prior to exposure in one method of the invention.

apparent from form of a non-photo- 2,940,847 Patented June 14, 1960 Fig.8 shows diagrammatically the arrangement ;of the color particles of Fig.7 after exposure.

Fig. 9 shows diagrammatically the arrangement of photoconductive colorparticles prior to exposure in another method of the invention.

Fig. 10 shows diagrammatically the arrangement of the color particles ofFig. 9 after exposure.

Fig. 11 shows diagrammatically the arrangement of non photoconductivecolor particles prior to exposure in still another method of theinvention. Fig. 12 shows diagrammatically the arrangement of the colorparticles of Fig. 11 after exposure.

Fig. 13 shows diagrammatically the relationship of elements for printingfrom the color image resulting in Fig. 12.

Fig. 14 shows diagrammatically the final arrangement the color particlesin Fig. 13 after exposure.

Fig. 15 shows one means employing color particles for producing animage.

Fig. 16 shows another means employing color particles for producing animage.

Fig. 17 shows still another means employing color particles forproducing an image.

The color particles shown in Figs. 1 to 6 are typical of the imageforming elements which can be utilized with the method of photographydescribed herein. In. the use of any of these particles the essentialaction is that in a layer of mixed color particles, those particles of agiven color will migrate or, if desirable, react oppositely by remainingunmoved when subjected to light of the given color. a I

The color particle 10 of Figs. 1 and 2 comprises one or more bits 12 ofa suitable photoconductor surrounded by a layer or coating 14 of dyedgelatin or similar material. Typical photoconductive materials includeselenium, zinc oxide, cadmium sulfide, cadmium telluride, anthracene,and sulfur. Actually any photoconductive powder may be used, and it ispreferred that the particle sizes fall in the range of 2 to 30 microns.The dyed layer may consist of any suitable dye in gelatin, wax, vinyl orsilicone resin, cellulose ester or similar material in a thickness offrom 4 to 25 microns. The powdered photoconductor may be mixed with thedyed layer material together with a solvent and then dried while beingagitated, as by a warm air blast. Spraying of the photoconductor-solvent-dye layer mixture into a heated chamber will alsoyield suitable particles. In order to increase the photographic speed ofthese particles it may be necessary to add to the dye layer a smallamount of a suitable salt to reduce the electrical resistance of thelayer and thereby permit more rapid charging of the particle.

Fig. 3 shows a particle 16 comprising a central core 18 which mayconsist of a clear, transparent glass or plastic head carrying atransparent photoconductive layer 20 and an outer, dyed, transparentlayer or coating 22. The multiple layers may be formed in the mannerdescribed in connection with particle .16, or the photoconductor layermay be evaporated onto the glass bead. A bead diameter of 3 to 30microns, a photoconductive layer of 5 to 60 microns thickness and a dyedlayer of 4 to 25 microns thickness will yield satisfactory particles inthe 21 to 250 micron diameter range.

or plastic bead 32 covered with a thin layer 34 of a conductingmaterial. In the case of glass beads a layer of fused transparent tinoxide is suitable. In thecase of plastic beads athin transparent layerof evaporated'metal is preferred, although treatment w th so calledanti-static solutions is also suitable. I 'As shown in Fig. 6 ,'it isalso possible to produce beads 36 which comprise a solid,

substantially spherical body 38 of tin'oxide or othertransfparent-electrically conducting materials which are suit ablycolored inthe mass. As in the case of other particles "-a diameter ofbetween 3 and 40 microns is preferred for particles of this class,although for some applications larger particles are suitable.

In: the practice of color electrophotography as set forth in the presentinvention the following series of :3 steps "or their equivalent must beper-formed in the approxim'ate'order shown:

(1) Establishment of an electrostatic field.

' (2) Production of a light image. a

(3) Charging or discharging of colored particles,

1 which are in the electrostatic field and on which the light image isreceived, in accordance with the color and pat- "ternof thelim'age.

(4) Migration of either the charged or the discharged particles of step3,-co'rre'sponding to the image, to a new surface or a new location.

' (5) Fixing or receiving onto a final support surface migristed ornon-migrated particles, or both, to form'a final fixed color image.

It will be noted'that with some arrangements to be delscribed certainsteps, particularly those enumerated 1 and). above, can be reversed inorder. 7

Figs. 7 and 8 show one arrangement whereby additive color images can beproduced by the use of colored photo-conductive particles. A glass orsimilar transparent plate 40 carries a transparent electricallyconductive layer 42 of NESA glass or thin evaporated metal. Anupperelectrode plate 44 spaced from plate and any suitable distance from 1 or2 millimeters to several centimeters, carries at its lower surface aparticle receiving layer 46 to be described below. A suitable source 48of DC.

3 voltage, connected to layer 42 and electrode 44 is provided withsuitable switching means 50. A negative potential of from 300 voltstoSOOO volts is applied to electrode 44 depending upon its spacing fromplate 40 and the characteristics of the particles employed. In thisarrangement particles 10 and. 16 of the type shown in Figs. "'1, 2 and3, colored red, green and'blue, are employed.

and may be also transferred to a black base. This may be the desiredimage if color reversal is a requirement of the process.

As shown in t e arrangement of Figs. 9 and 10 this method can also beadapted to subtractive color photography. 'Here the arrangement ofelectrodes is similar to that of Figs. 7 and 8 and the parts have beennumbered correspondingly. in this modification, transparent'coloredparticles 52 of the type shown in Fig. 4 are employed, containing cyan,magenta and yellow dye at centers 28 and colored red, green and bluerespectively at layers 32. Initially particles 52 are randomlydistributed on surface 42 in a layer 1m 4 particles deep, although inFig. 9 they are-shown in a single layer with reguiar distribution forthe purpose of explanation. When subjected to a light image some of theparticles will be moved upwardly depending upon the color of light.Where red strikes a red jacketed particle 24 containing cyan dye restingon surface 42 the resistance of the photoconductive layer, is reduced,the particle becomes charged and migrates to surface 46. Thephotoconductive layer of a blue jacketed yellow containing particlestruck by red light will remain unchanged in electrical resistance andwill not acquire a charge from surface 42 and will not migrate. Neitherwill a green jacketed magenta containing particle migrate when struck byred light. Blue light will'cause blue jacketed yellow containingparticles to migrate while leaving the red and green jacketed particlesunmoved, and green light will cause green jacketed magenta containingparticles to migrate while leaving the similarsheet is laid over theparticle carrying surface, ab-

sorbent surface in contact with the particles, and the resultingsandwich' subjectcd to pressure as, for example, by passing between apair of rollers. The pressure causes the particlesto burst, and the dyepreviously contained within them is absorbed by the absorbent surface ofthe "base material. The two base layers are stripped apart and A-substantially'uniform layer of these particles is placed on conductingglass 42. For purposes of illustration a single layer is shown in Fig.7; the layer may be 3 or 5 more particles deep, and can be applied bysimply cascad- -ing-on'to surface 42, or, by spraying, or by applyingwith a roller.

After exposure to light of various colors the particles migrate to theposition shown in Fig. 8. Where red, green and blue light reach surface42, the red, green and blue particles, respectively, migrate to surface46. Where white light reaches surface 42 particles of all three colorsmigrate, while at the non-illuminated areas there is no migration ofparticles. The image which results on surface is a positive image of theadditive type, which for proper viewing as a print must be transferredto a black base, for example a sheet of black surfaced paper orplasticmaterial. The image may be transferred to such a base by theusual'electrostatic means or through action of a suitable adhesivelayer. Fixing of the image may be accomplished by heating the surface tocause fusion and bonding of the particle surface or the particles may beimmobilized by means of a transparent adhesive overlay.

If desired, surface 46 may constitute an adhesive layer surface 42 is anegative image, also of the additive type the particles of debrisremoved by means of brushing or washing. The resulting image will be acolor reversal of the original.

If the transfer is made. from surface 42 after exposure the resultingvcolor image will be a positive color photograph.

Figs. 11 to 14 illustrate still another way in which the invention maybe employed to produce subtractive color images. Here the particles 34are non photoconductive and possess the characteristitcsdescribed inconnection with Figs. 5 and 6. A layer of particles 34, 3 or 4 deep, isdeposited on a-photoconductive layer 60 of selenium or other suitablematerial supported on an electrically conducting basev 62 of brass,aluminum or the like wln'ch 'ticles is exposed by light passingdownwardly through is connected to one terminal of a voltage source suchas described in connection with Fig. 7. Spaced from and parallel to thesurface 60 is a sheet of glass 64 carrying at its under side a layer 66of transparent electrically conducting material which is connected tothe second terminal of the high voltage source. The layer of parsheet 64and conductive layer 66 onto particles 34. For a given color or spectralband of exposing light one or more of head colors, which are cyanmagenta or yellow,

will transmit the light to the selenium layer below. The

. selenium layer thereupon becomesconducting, the transmitting beadbecomes eharged and migrates upwardly to layer 66..

layer form it As shown in Fig. 12, red, green and blue light 'exposureresults in migration of magenta and yellow, cyan and yellow, and cyan,and magenta particle's, respectively, to form subtractive color layers.In order to assure that the particles apply themselvesin substantiallyis preferred that the sizes and conductivities of the particles becontrolled. By making the cyan particles somewhat smaller and utilizinga relatively lower conductivitysurface 38 over the core, these particleswill migrate first to form a cyan layer. By increasing the size of themagenta particles and increasing :the electrical resistivity of theirsurface the magenta particles will migrate next to form the-secondlayer. Preferably the yellow particles are the largest and possess thehighest resistivity, thereby being deposited last. While thediagr-ammatic representation in the drawing has been that of a singlelayer for the sake of simplicity, it should be borne in mind thatmultiple layers of the type described represent the actual structure.Where white light reaches the particle layer all particles migrate tosurface 66.

Where no light strikes the particle layer no particles migrate.

The intermediate image appearing on surface 66 is next used for printingas shown in Figs. 13 and 14. The image on surface 66 is projected onto aselenium plate 7072, similar to plate 6062, through a transparentconducting plate 74--76 similar to plate 64-66 onto color particles 34.During exposure these particles migrate to surface 76 to form a.subtractive color image corresponding to the original subject of Fig 11.

Fig. :15 shows one means for employing color particles for theproduction of color photographs. A transparent plate 44 carrying aconductive layer 46, such as shown in Figs. 7 to is spaced away from andparallel to a grid 80. Layer 46 and grid 80 are maintained at a suitablepotential ditlerence by connection to a source 48 of high voltage.Spaced away from the opposite surface of grid 80 and parallel thereto isa particle distribution head indicated generally at 82, consisting ofspaced apart plates 84 which form a series of alternate duct areas 86and 88. Duct areas 86 are connected to a supply chamber 90 while ducts88 are connected to a return chamber 92. An air blast shown by arrow 94carries mixed color particles, such as those shown in Figs. 1, 2 and 3,into duct areas 86, through screen 80 and against layer 46 in adirection generally perpendicular to the latter. The appropriatelycolored particles are charged by their passage through the grid andadhere to layer 46. Particles which remain inactive, because of theirnon-response to light of a wavelength to which their resistance remainsunchanged, are drawn into duct areas 88, through chamber 92, andreturned as indicated by arrow 96 to a receiving chamber, not shown. Thecolor particles on layer 46 are transferred to a black paper or plasticbase and are there fixed by heat or other well known means.

Fig. 16 shows diagrammatically a continuous color pn'nt machineemploying color particles. An endless belt 100 of electricallyconducting flexible material carries on its outer surface a layer 102 ofselenium or other photosemiconductor and is supported by a pair ofpulleys 104. Spaced parallel to and spaced from belt 100 is a web ofsuitable base material 106 such as transparent plastic fed from roll 108under rollers 116 and onto takeup roll 112. One terminal of a voltagesource 48 connects to belt 100 and the other to a transparent electrode114 in contact with base material 106. An exposure station 116 islocated above electrode 114; the image at 116 moves synchronously withbelt 100 and web 106. 7

Layer 102 receives a charge of color particles such as that shown inFig. 5 from a hopper-like distributor 118 which cascades the particlesonto the belt, the angle of repose of the latter being such that only asufiicient depth of particles is retained, the remainder being carriedaway for reuse through duct .120. Layer 102 is cleaned of unusedparticles by means of a rotary brush 122, and the unused particles areretained in chamber 124 for reclassification and reuse. Reclassificationof the particles into three portions, each containing a single color isaccomplished in a fashion analogous to the color process itself, i.e. bysuccessive exposure of the particles, while on a photoconductivesurface, to light of a given color.

The continuous printershown in Fig. 17 employs a rotary transparent drumcarrying on its outer surface a transparent conductive coating 132.Within the drum is an exposure station comprising a light source 134,condensers 136 and projection lens 138. The trans parency to be printedis shown in the form of a web 140 passing under the condenser and movingsynchronously with drum 130 onto takeup spool #142. The color particlesemployed are of the type shown in Fig. 4 and are suspended in a liquiddielectric such as light mineral oil or carbon tetrachloride, themixture 144 of particles and liquid vehicle being carried in a sumpcompartment 146. A high voltage source 48 connects to the conductivelayer 132 and to a fine mesh grid 148 beneath the mixture 144 and spacedfrom 0.1 to 10 mm. from layer 132 depending upon particle size andconcentration as Well as potential. An air squeegee 150 directs airagainst the surface of the drum to remove unwanted particles and towholly or partially dry the drum surface.

A web of transparent plastic base material 152 is held in contact withthe drum by means of a roller 154 and passes under a. suitable fixingstation 156 before being taken up on reel 158. A brush 160 cleans thedrum surface prior to exposure. An ultrasonic generator 162 is locatedin the developer sump below grid 148 and causes the impingement ofdeveloper particles against the drum in a direction substantially normalto the drum surface.

In operation, the original on film 140 is imaged through the drum andonto the layer of particles between surface 132 and the grid 148, theimage moving at the same speed as the periphery of the drum. Particlesof the correct color are energized in accordance with the showing inFigs. 9 and 10 and adhere to coating 132. Accidentally entrainedparticles are removed by the air jet 150 and the color image istransferred to base 152 and fixed by heat or similar means at station156. The cleaning action of brush 160 insures that surface 132 is freeof contaminating particles prior to exposure. From time to time theproper relationship of relative concentration of the color particles inthe mixture 144 must be restored by the addition of the needed color orcolors.

It is apparent that other arrangements for creating an electrostaticfield and for causing selection of color particles according to theexposing color are readily possible and that other constructions of thecolor particles themselves are practical and could be devised by thoseskilled in the art. The applications are not limited to photographyalone, the method lending itself well, for example, to the printing ofposters, signs, labels and reflective traflic signs.

I claim:

.1. A method for photographically reproducing images in natural colorwhich comprises providing a uniform layer of developer particles of atleast two primary colors resting upon one of a pair of parallel,spaced-apart,

planar electrodes at least one of which is transparent to light withinthe visible spectrum, establishing an electromotive potential betweensaid two electrodes, projecting an image pattern containing at least thesaid two of the primary colors through said transparent electrode and tosaid developer particles thereby causing the developer particles toselectively migrate to the opposite electrode in accordance with thecolors in the image pattern, said developer particles comprising aphotoconductive material completely coated with an electricallyinsulating filter layer which filter layer is transparent to only one ofsaid primary colors, the spacing of said electrodes and the,

magnitude of saidpotential being sufficient to cause said particles toadhere to said opposite electrode in accordance with the image pattern,said migration being a result of the increase in electrical conductivityof said'photoconductive material.

2. The method of claim 1, wherein said developer particles consist of asingle photoconductive particle coated with the filter layer.

3. The method of claim 1, wherein said developer particles consist of apluraiity of photoconductive particles embedded in the filter layer.

4. The method of claim 1, wherein said developer particle comprises acentral, substantially spherical core of insulating material completelycoated with the photoconductivefmaterial which is in turn coated withthe filter layer. a

5. The method of claim 1, wherein said developer particle comprises acapsule containing a liquid dye of a color complementary to that of thefilter layer, said capsule being completely coated with thephotoconductive material which is in turn coated with the filter layer.

6. The method of claim 1, including the further step of transferring theparticle pattern adhering to said opposite electrode to-a final imagereceiving surface in.

References Cited in the file of this patent UNITED STATES PATENTS 71,996,928 Mannes et al. Apr. 9, 19.35 2,277,393 Depew Mar. 24, 19422,297,691 Carlson Oct. 6, 1942 2,752,833 Jacob July 3, 1956 2,753,308Landrigan Julyr3, 1956 2,758,939 Sugarman Aug. 14, 1956 2,781,704 Mayoet a1. -4"; Feb. 19, 1957 2,807,233 Fitch Sept. 24, 1957 2,808,328 JacobOctal, 1957 2,817,277 Bogdonoif Dec. 24, 1957 2,845,348 Kal lm-an July29,

OTHER REFERENCES V Luther: Possible Methods of Colour Photography," TheBritish Journal of Photography, vol. V, #51, March 1911, pages 17--19.

1. A METHOD FOR PHOTOGRAPHICALLY REPRODUCING IMAGES IN NATURAL COLORWHICH COMPRISES PROVIDING A UNIFORM LAYER OF DEVELOPER PARTICLES OF ATLEAST TWO PRIMARY COLORS RESTING UPON ONE OF A PAIR OF PARALLEL,SPACED-APART, PLANAR ELECTRODES AT LEAST ONE OF WHICH IS TRANSPARENT TOLIGHT WITHIN THE VISIBLE SPECTRUM, ESTABLISHING AN ELECTROMOTIVEPOTENTIAL BETWEEN SAID TWO ELECTRODES, PROJECTING AN IMAGE PATTERNCONTAINING AT LEAST THE SAID TWO OF THE PRIMARY COLORS THROUGH SAIDTRANSPARENT ELECTRODE AND TO SAID DEVELOPER PARTICLES THEREBY CAUSINGTHE DEVELOPER PARTICLES TO SELECTIVELY MIGRATE TO THE OPPOSITE ELECTRODE